Staircase-Like Crack Progression Due to Hydrogen Embrittlement of Cold-Worked Steel Strand

Stressed carbon steel strand in an ungrouted duct susceptibility to pitting corrosion is low due to surface corrosion, but susceptibility of steel strand to Hydrogen Embrittlement (HE) can increase under those conditions. The HE will facilitate crack growth within the strand. Various crack propagation mechanisms, such as longitudinal splitting and shear-cracking, have been shown as possible strand failure mechanisms by themselves in strand, but this may not be true in stressed strand in piles that has been embrittled by Hydrogen and without pre-cracking (Cracks initiating from stress concentrations naturally rather than with notching). Concentration measurements were performed to determine the level of Hydrogen involved in the embrittlement. Results indicate that the fracture mechanism differs from shear cracking or longitudinal splitting alone as previously shown, but is a multi-step process of crack propagation starting perpendicular to stress, followed by variations of inter-lamellae longitudinal splitting at brittle region of lamellae and shear cracking at breaks in the lamellae. This process results in the crack following a “staircase” progression, and finally leading to ductile overload once cross-section has been significantly reduced. This fracture mechanism was also shown to be valid whether the strand was stressed by bending or mult-axially by stressing through a duct.